Process for optimizing the process of copper electro-winning and electro-refining by superimposing a sinusoidal current over a continuous current

ABSTRACT

This invention discloses a process of the electro-winning of metals, in particular copper, is done in a strongly acidic aqueous solution (˜180 gpl H 2 SO 4 ), with permanent cathodes of stainless steel or Cu starting sheets, and Pb—Ca—Sn anodes, in tradition electrolysis cells to obtain copper cathodes wherein in the electrolytic cells, a sinusoidal current is super-imposed over the continuous current in order to produce high-purity electrolytic copper. This invention also discloses the electro-refining process of metals, in particular copper, is done in a strongly acidic aqueous solution (˜180 gpl H 2 SO 4 ), with permanent cathodes of stainless steel or Cu starting sheets, and anodes of impure copper from founding, in traditional electrolysis cells in order to obtain copper cathodes wherein in the electrolytic cells, a sinusoidal current is super-imposed over the continuous current in order to produce high-purity electrolytic copper.

This application is a Continuation of U.S. Ser. No. 11/599,727, filed 14Nov. 2006, which claims benefit of Serial No. 2963-2005, filed 14 Nov.2005 in Chile and which applications are incorporated herein byreference. To the extent appropriate, a claim of priority is made toeach of the above disclosed applications.

OBJECT OF THE INVENTION

The present invention optimizes the processes of metal electro-winningand electro-refining (in particular copper) by superimposing asinusoidal current over a continuous current.

This invention consists of superimposing a sinusoidal current over thecontinuous current presently used for operations. This improves theprocesses of electro-winning (EW) and electro-refining (ER) of metals,particularly copper. Depending on the amplitude and frequency selectedfor the sinusoidal signal, the alternative current component exercises aspecific action on the electrochemical double layer of the electrodes[(+) anodes and (−) cathodes] in the electrolysis cell. Theelectrochemical double layer is charged and discharged during thesinusoidal cycles on both internal and external Helmolthz planes and thediffuse layer, noticeably improving the movement and displacement of theionic species that are generated or consumed at theelectrode/electrolyte interface. From a phenomenological point of view,the use of superimposed current signals makes the double layer and thediffuse layer behave like a true hydraulic pump, forcing ionic flows toand from the electrodes in a zone that is inaccessible when using forcedconvection (electrolyte flow to cells). During the use of continuouscurrent signals, only the concentration gradients, assisted by thesolution temperature, generate movement by diffusion of the determinantions for the electrochemical reactions and the chemical reactions thattake place around the electrodes.

We have proven experimentally that the superimposition of a sinusoidalcurrent signal (amplitude 400-600 A/m²; frequency 1,000-10,000 Hz) overthe continuous current acts on the electrochemical double layer,enormously benefiting the electrolysis processes, especially theelectro-winning and electro-refining of copper. This new techniqueallows the use of higher DC current densities in conventional copper EWand ER cells (>300 A/m²), the equivalent of an increase in productionequal to the installed capacity of an industrial plant. Moreover, theinvention improves the physical-chemical cathode quality, theelectrochemical corrosion behaviour of the Pb—Ca—Sn anodes, and theelectrochemical passivation behaviour of the Cu anodes in the ERelectrolysis cells. Consequently, additives are not required to even outthe crystalline growth, nor are they necessary for anodic corrosion.

BACKGROUND Description of the Knowledge of the Material

The Copper Electro-Winning (EW) Process

Copper has been recovered industrially and commercially from Cu²⁺solutions using the combined LX-SX-EW process for some 30 years. Thistechnique has sustained increased copper production thanks to thetechnical feasibility of purifying and concentrating Cu²⁺ solutions withextraction technology using solvents (SX). High-purity cathodes areproduced in the EW tank-house. This global process has the addedadvantages of low capital and operational costs.

The chemical quality of the copper cathodes obtained through LX-SX-EWreaches a degree of purity of “five nines” (99.999%), with extremely lowlevels of impurities (e.g., sulphur, lead, oxygen, iron, hydrogen,carbon). In general, cathode production at most LX-SX-EW plants is over90% “high-grade”, or superior in quality to Grade A as defined by theLondon Metal Exchange (LME). At present, the Chilean copper productioncapacity through LX-SX-EW exceeds 3.0 million metric tons per year.

The objective of copper electro-winning plants is to produce thegreatest number of high chemical and physical purity metal cathodes,with the lowest possible specific energy consumption. The plants rely onexternal electrical energy, and production is determined based on thelevel of the imposed continuous DC current. This process is carried outin a strongly acidic aqueous solution (˜180 gpl H₂SO₄), with permanentstainless steel cathodes or Cu starting sheets and Pb—Ca—Sn anodes. Themain electrochemical reactions are given in Table 1.

TABLE 1 Main electrochemical reactions in the EW of Cu REACTIONS V°(Volts/SHE) On the cathode: Cu²⁺ + 2e⁻ = Cu 0.34 On the anode: H₂O =½O₂ + 2H⁺ + 2 e⁻ 1.23

Table 2 indicates the secondary electrochemical reactions that can alsooccur on the electrodes. Since the cell potential is between 1.8 and 2.2V, from a thermodynamic point of view, practically all the reactionsindicated in Tables 1 and 2 can take place on the electrodes.

TABLE 2 Secondary reactions in the EW of Cu REACTIONS V° (Volts/ESH) Onthe cathode: 2H⁺ + 2 e⁻ = H₂ 0.00 On the anode: Pb + SO⁴⁻ = PbSO₄ + 2 e⁻−0.36 Pb²⁺ + 2H₂O = PbO₂ + 4H⁺ + 2 e⁻ 1.46 Mn²⁺ + 4H₂O = MnO₄ ⁻ + 8H⁺ +5 e⁻ 1.51 2Cl⁻ = Cl₂ + 2 e⁻ 1.36 Co²⁺ = Co³⁺ + e⁻ 1.82

In order to reconcile cathode production with chemical quality, copperEW plants operate with electrical DC current density levels thatoscillate, by design, around 300 A/m², with 92-94% current efficiency,and with specific energy consumptions of 1,700-2,000 kWh/ton. Someoperational variables can be manipulated externally, including theconcentration of cupric ions in the solution entering the cells, theelectrolyte temperature, and the electrolyte flow into the cell; theseshould be established in function of the imposed DC current and thedesired chemical quality. The variable ranges are as follows:

1) Current density: 160-350 A/m², continuous, DC,2) Flow of electrolytes to cell: 240-340 l/min/cell,3) Electrolyte distribution: conventional or bottom of cell/manifold,4) Cathode-cathode distance: 100-120 mm,5) Cathodic cycle: 5-7 days,

6) Temperature: 40-50° C.,

7) Laminated Pb—Ca—Sn anodes: 6-7.5 or 9 mm,8) Permanent stainless steel cathodes: 316 L, 3-3,3 mm,9) Additives: “guar-gum”, FeSO₄, and CoSO₄,

The Copper Electro-Refining (ER) Process

Copper electro-refining (ER) is an electrochemical process for obtaininghigh purity copper cathodes corresponding to the last stage in thetreatment of sulphide copper ores.

In Chile, more than 40% of cathodic copper is obtained by ER,specifically in the Codelco Chile refineries (Chuquicamata and SalvadorDivisions) and in the Ventanas electrolytic copper refinery, belongingto the company Empresa Nacional de Minería (ENAMI; National MiningCompany).

This process combines the electrochemical dissolution of impure copperanodes (97-98%) with copper reduction in order to produce high puritycathodes (% Cu>99.99+).

In general, these copper refineries operate within a density current(DC) range of 200-350 A/m², with voltage drops of 0.25-0.30 VDC. Thecopper concentration in the solution varies between 40 and 60 gr/lt. Thesulphuric acid concentration in most ER plants fluctuates between 170and 220 gr/lt, for electrolyte temperatures of 50-70° C. In copperrefineries, the electrolyte flow generally varies between 15 and 30(lt/min/cell).

In order to regulate the crystalline growth on the cathodes, chemicalagents are added during the ER of copper. The most common additives inthis process are Thiourea, glue, Avitone, and chlorhydric acid, allsoluble in the acid electrolyte and dosed in concentrations on the orderof ppm or gr/ton of copper produced.

The projections of most new refineries call for permanent cathodetechnology that can operate at high current densities (I>250 A/m²).Nonetheless, these plants must maintain high operational standards inorder to assure that the physical and chemical quality of the cathodesis adequate:

-   -   Good physical shape of anodes and cathodes, without loss of        verticality or plenitude, and with an adequate chemical        composition,    -   Electrolysis cells that allow good electrolyte circulation,    -   Adequate purification process for the solutions,    -   Good state of the electrical contacts between the electrodes and        the equipotential bars,    -   Rigorous control of current leaks and short-circuits.

Anodic Impurities

In the copper electro-refining process, the chemical quality of thecommercial cathodes is largely conditioned by the impurities in theanodes from founding. Impurities (e.g., selenium, tellurium, bismuth,antimony, arsenic, etc.) are incorporated into the electrolyte asdissolved species, colloids, or solids that remain in the solution orare gradually incorporated onto the anodic slime that is deposited atthe bottom of the cell.

Under normal operating conditions, arsenic enters the electrolyte asAs(III) and is oxidised to As(V) by dissolved oxygen. The affect of theAs(III) present in the electrolyte on the cathodic process is notnegative; rather, the arsenic prevents the formation of floating slimeto the degree that it is found incorporated into the electrolyte inexcess with respect to the other impurities, in particular, antimony andbismuth. The antimony on the anode normally hydrolyzes to Sb₂O₃ and,given an excess of As(V), is able to form compounds of the typeAs₂O₅*Sb₂O₃.

Both these species have been reported as colloidal types, with lowdensities and a clear tendency to remain suspended in the solution,constituting one of the most important sources of cathodiccontamination. Other compounds reported in the literature that arelikely to contaminate the copper cathode are Cu₃As, 3Cu₂O*4NiO*As₂O₅,and 3Cu₂O*4NiO*Sb₂O₃. This last compound, denominated “kupferglimer”,does not dissolve chemically or electrochemically, and is one of themain species giving rise to floating slime.

Industrial experience indicates that the impurities found in commercialcathodes come mostly from the incorporation, by occlusion, of the anodicbar and the electrolyte and, to a much lesser degree, by theco-deposition of the electrolytes of these species.

Copper Cathode Reduction

The industrial process of copper cathode reduction uses a continuouselectrical current and involves electrochemical reactions that takeplace at the electrode-solution interface over both the cathodes andanodes.

The simplest way to represent the electrode processes in an electrolysiscell is through an electrical circuit equivalent to that shown inFIG. 1. The equivalent electrical circuit considers three elements (Rs,C_(dc), Z_(f)) from the cathodic and anodic sides.

The faradic impedance is proper to each electrode process. In general,it is necessary to rely on phenomenological models to characterize thekinetic or the velocity of the reaction. This is generally obtained byestablishing reaction schemes.

Electrochemical Kinetics of the Cathodic Reaction

Whether by EW or ER of copper in industrial plants, one of the mostrecurrent mechanisms for characterizing the cathodic reaction of copperover a substrate of copper, titanium, or stainless steel, is thefollowing:

$\begin{matrix}{{{Cu}_{0}^{+ 2}\overset{D/\delta}{->}{Cu}^{+ 2}},} & ( {{stage}\mspace{14mu} 1} ) \\{{{{Cu}^{+ 2} + e}\underset{k_{2}}{\overset{k_{1}}{rightarrow}}{Cu}^{+}},} & ( {{stage}\mspace{14mu} 2} ) \\{{{Cu}^{+} + e}\underset{k_{4}}{\overset{k_{3}}{rightarrow}}{{Cu}.}} & ( {{stage}\mspace{14mu} 3} )\end{matrix}$

In this case, 3 kinetic stages make up the reaction leading from thereactive (Cu⁺² _(o)) to the product (metallic Cu on the cathode): thetransport of cupric ions by diffusion (stage 1) and two successivemono-electronic stages of electron transfer (stages 2 and 3).

In industrial plants operating under a stationary regimen withcontinuous CD current signals, the mathematical model that characterizesthe reaction scheme is as follows:

$\frac{i_{f}}{F} = \frac{2*( {{k_{2}*k_{4}*^{2*b*V}} - {k_{1}*k_{3}*\lbrack {Cu}_{0}^{+ 2} \rbrack}} )}{{k_{2}*^{{({1 + \beta_{2}})}*b*V}} + {k_{3}*^{\beta_{1}*b*V}} + \frac{k_{3}*k_{1}}{D/\delta}}$

where:k₁=kinetic constant of the first ionic exchange, direct, (cm/s);k₂=kinetic constant of the first ionic exchange, inverse, (cm/s);k₃=kinetic constant of the first ionic exchange, direct, (cm/s);k₄=kinetic constant of the first ionic exchange, inverse, (mol/cm²/s);β₁ and β₂=charge transfer coefficients, a-dimensional;[Cu⁺² ₀]=Cu⁺² ₀ concentration, (g/cm³).

$b = {\frac{F}{R*T}( {Volts}^{- 1} )}$

where:R=universal constant of the gases, (cal/mol/° K);F=Faraday constant, (cb/eq);T=absolute temperature, (° K);V=electrode potential, (Volts);D=Cu⁺² diffusion coefficient, (cm²/s);δ=thickness of the limit layer, (cm);i_(f)=faradic current, (A/cm²).

Table 3 presents the characteristic values of the model's parameters,validated on laboratory and industrial scales, for the kinetic of thecopper cathode reduction reaction. The data obtained suggest that whenthe cathodic super-potential values are low (near equilibrium), thekinetic of the copper cathode reduction is controlled by a mixed regimenof charge transfer (stage 2) and diffusion (stage 1); the mechanism istypified as a slow kinetic.

TABLE 3 Values of the constants from the copper cathode reduction modelConstant Numerical value Unit of measurement k₁ 3.46 * 10⁻⁶ cm/s k₂1.42 * 10⁻⁸ cm/s k₃ 5.82 * 10⁺⁶ cm/s k₄ 3.49 * 10⁻² mol/cm² * s D/δ1.24 * 10⁻³ cm/s β₁ 0.5 — β₂ 0.5 — Vo +50 mV/ESH R 1.987 cal/mol * ° K F96495 cb/eq

FIG. 2 shows the characteristic stationary i=f(V) of copper cathodereduction and the current imposed for industrial copper EW and/or ERoperations through the point of functioning (V_(o), l_(o)) on the curvei=f(V). In order to assure that the chemical quality of the cathodes isgood, this point (V_(o), l_(o)) should be located in a kinetic controlzone due to charge transfers, taking care with the productionindicators.

Electrochemical Kinetics of the Anodic Reaction

In the EW of Cu, the main anodic reaction is water oxidation, asindicated in Table 1. Since the oxidation of the water over the Pb—Ca—Snanode is slow, its kinetic is controlled by the charge or electrontransfer stages.

H₂O=1/2O₂+2H⁺+2e ⁻

The anodic process continuously generates the movement of protons (H⁺)from the reaction interface toward the bulk. This plays a fundamentalrole in the electrochemical passivation behaviour of the Pb—Ca—Sn anodeand, consequently, electrode breakdown by corrosion. Thus, the capacityof the anode/electrolyte interface to impede the chemical precipitationof lead sulphate (the compound responsible for the generation of passivezones on the copper electro-winning anodes) depends on the movement ofthe H⁺ ions from the electrode (where they are generated) toward thesolution. Continuous current signals do not impede the accumulation ofthese cations produced during water oxidation, and only theconcentration gradient and electrolyte temperature are responsible forgenerating the movement of charged species toward or from the electrode.When operating conditions are inadequate (low temperature, inadequateelectro-active species concentration, electrolyte flow to a deficientcell), the H⁺ ions move very slowly from the reaction interface towardthe bulk, where the chemical precipitation of the lead sulphate iscatalyzed over the anode surface. This tends to block the surface sitesfrom the anodic reactions, causing anodic corrosion of the Pb—Ca—Snelectrode. The resulting permanent anode corrosion takes the form ofscales that are deposited at the cell bottoms, constituting “leadslime”, which should be removed every 3 or 4 months.

Otherwise, the main anodic reaction in the ER of Cu is the oxidation ofthe impure Cu anode that comes from founding (see Table 3). The kineticof the slow copper oxidation is controlled by the stages of chargetransfer and ion diffusion.

Cu=Cu²⁺+2e ⁻

The oxidative process of the anode should impede the accumulation ofanions and cations at the anode/electrolyte interface. This increasesthe possibility of a precipitation of mixed oxides (e.g., of As, Bi,Sb), as these can cause passivation of the refinery anodes, inparticular when operating with elevated current densities (i>300 A/m²).During copper electro-refining, it is imperative to avoid the phenomenonof anode passivation, which requires that operations in the affectedcell sections be stopped, with the consequent losses of production, andmetallic efficiency, and energy efficiency in the copperelectro-refining process.

Operational Problems Affecting Cu Cathode Production and Quality(Current Technology)

The most important variables affecting cathode production and qualityare the following:

-   -   Imposed current,    -   Electrolyte temperature,    -   Concentration of ions in the electrolyte,    -   Flow of electrolytes to the cell, and    -   Electrochemical behaviour of the electrodes (cathodes and        anodes).

In the industrial processes of Cu electrolysis, the key to increasedproduction (given the same installed capacity) is increased currentdensity. However, with the current cell designs and technology,indiscriminate increments in current density decrease thephysical-chemical quality of the cathodes. As already indicated, copperEW and ER plants generally operate with current levels between 240 and350 A/m², not being able to surpass 320 A/m² in order to safeguard thequality standards of the cathodes.

An increase of current density in conventional electrolysis cells mustconsider the following technical and economic aspects:

-   -   Appropriate technology for increasing the current density        without experiencing a deterioration of the process control        indicators, most notably the physical-chemical quality of the        cathodes, current efficiency, and specific energy consumption;    -   An anode material able to withstand elevated currents without        experiencing significant corrosion;    -   Control of the current passage through the intercell        electrode/bar contact;    -   Control and removal of acid mist in the EW of Cu;    -   Control and removal of lead slime in the EW of Cu;    -   Control of anodic passivation in the ER of Cu.

Recent Technological Developments

The most important technological developments for improved electrolysisprocesses in the electrometallurgy of copper from the point of view ofproduction and cathode quality are the following:

Permanent Cathode Technology

This technology has been applied worldwide for the last 10 years incopper EW and ER plants, providing important benefits such as operatingat greater current densities (up to 350 A/m² in some plants) and greaterproductivity indices.

Intercell Equipotential Bars

New intercell bar designs such as those used by Asturiana de Cinc intheir copper EW plants (Compañia Minera Doña Inés de Collahuasi andCODELCO Norte Planta RT, Chile) have reduced voltage by up to 10 mV/cellwith respect to a triangular base design. Considering the parallel-typedistribution of the current in the cell electrodes, new bar designs havebeen patented recently by the companies “Outokumpu” (Finland) and“Optibar” (Chile). These novel designs offer greater control over thecurrent distribution in the electrolysis cells and their inventors findthat this improves the control indicators for the productive processes.It should be noted that this invention has not been applied massively incopper electrolysis plants.

Insoluble Anodes in the EW of Cu

One of the most relevant problems in the copper EW process is related tothe material of the anode body and the technology and design of theelectrode bar/body union. This physical union between the copper bar andthe catalyzing Pb—Ca—Sn plate that makes up the anode body is the sitefor water oxidation. Several types of anodes have been developed thatincorporate oxides of noble metals over the lead surface [ALE anodes(lead anodes covered by oxides of noble metals) and DSA anodes (titaniumanodes covered by oxides of titanium and ruthenium)], in order tocatalyze better the water oxidation reaction. Another developmentconsists of the so-called MOL (mesh on lead) anodes, made byEltechsystem Co. (USA). In DEA anodes, a Ti mesh covered by oxides ofnoble metals such as Ru and/or Ir is placed over the conventional plateof the Pb anode.

It should be noted that, in general, these developments have notpresented encouraging results for large-scale incorporation into theproductive copper EW processes due to inadequate designs, elevatedcosts, and poor adherence of the noble metal oxide films that catalyzethe water oxidation reaction.

New Electrolysis Cells

The most relevant technological innovations in the field have recentlyresulted in the design of more compact cells for the electro-winning ofcopper. One such innovation is the EMEW process (an English abbreviationthat commercially defines the integral process that encompasses cells,hydraulic systems, piping, rectifiers, electrodes, and mechanicalsystems for copper cathode harvesting). In the EMEW process, theelectro-deposition of copper is done on closed cylindrical cells, withtubular stainless steel cathodes and DSA anodes (titanium anodes coveredby titanium and ruthenium oxides). The cell operates at currentdensities >600 A/m² and has been used industrially to treat diluted Cu²⁺solutions, resulting in cathodes of high chemical quality and noenvironmental pollution by acid mist.

Control of Acid Mist in Copper EW Plants

At current densities >200 A/m², the generation of acid mist in copperelectro-winning cells requires the implementation of appropriate stepsfor its control. Hoods or ventilated covers placed over the cell andforced air (DESOM process) have given good operational results, as hasthe addition of tensoactive chemical antifoam agents such as FC 1100(3M) or small amounts of “quillay” extract (<5 ppm) to the electrolytein the copper electro-winning cells.

Increased Operating Current in Copper EW Plants

Two main techniques have been used to increase the operating currentdensity in the copper electro-winning plants:

-   -   Ultrasound vibration, and    -   Agitation through the injection of pressurized air (air        sparging).

Of these alternatives, only air sparging at the limit layer of diffusionhas shown possibilities for application in the electro-winning ofcopper, according to the test programs of electrolysis with agitation byair carried out by Kennecott and Inco, amongst others. Air sparging isbeing developed for larger scales and a reliable air sparging designthat allows operating at densities over 600 A/m² is expected to beavailable in the medium term. This technology should decrease cellvoltage in 400-600 mV, with the consequent reduction in the plant'senergy consumption. Nevertheless, this technological solution requiresalternative anode technology, as the Pb—Ca—Sn anodes cannot withstand600 A/m² of continuous current density without mechanical and corrosivedeterioration in the cells, to the detriment of the cathodic quality.

Periodic Current Inversion PRC, in Processes of the ER of Cu

This technology was developed for the processes of the ER of copper.Even though it is rarely applied in copper electrolytic refineriesaround the world, this unique technology requires little time for theelectrode to change polarity. PRC allows the cells to operate atelevated current densities, and the inversion of the current controlsthe preferential growth of the cathodic deposits and avoids theformation of a-circular nodules and crystals that cause short-circuitsin the electrolysis cells.

Very few copper electrolyte refineries in the world still use thistechnology in their plants.

Use of Super-Imposed DC+AC Current Signals

This type of super-imposed alternating signal at the continuous levelhas been used successfully in electroplating processes on sensitizedsurfaces of materials that are not conductive for currents (e.g.,pressed circuit plates, plates for electronic components, or diversegeometric PVC bases metallized for practical and decorative uses inautomobiles). These processes seek very fine, smooth electrolyticdeposits of copper and chrome that are very adherent to the basesurface. In fact, these processes are reported to benefit thesuper-imposed sinusoidal signal at the continuous level in terms of thequality of the metallic deposit by acting directly on theelectrochemical double layer, although no further phenomenologicalexplanations are given.

On the other hand, information was recently posted on-line regarding aninvention patent developed in Europe that concerns the increased usefullife of automobile batteries that use lead, lead dioxide, sulphuricacid, and water as principle components. The invention is supported bythe development of an electronic component that, coupled to theelectrical system of the automobile, allows the battery current to befed with sinusoids during the charging process. This technology has beenshown to improve the useful life of the battery considerably. Althoughthe article does not present the phenomenology of the invention;nevertheless, it can be inferred that the sinusoid-type current signalssuper-imposed over the continuous level allow greater regeneration ofthe products on the battery plates, by the action of the alternatingcomponent on the electrode phenomena.

Description of the Proposed Technology

The technology proposed for the present invention consists ofsuperimposing a sinusoidal AC signal of constant frequency and amplitudeover the continuous DC signal and applying this to the processes ofelectro-winning and electro-refining of metals, in particular copper, asshown in FIG. 3. The amplitude and frequency of the alternating signaldepend on the faradic impedance of the cathodic and anodic processesdescribed in FIG. 1.

The range of the electrical variables applied to the cathodic reductionprocesses of copper using conventional vs. the proposed technology ispresented in Table 4.

TABLE 4 Range of electrical current applied in the cathodic reduction ofcopper: conventional vs. proposed technology Range of electrical currentapplied in Range of electrical current applied the cathodic reduction ofcopper. in the cathodic reduction of copper Conventional technologyProposed technology DC: 240-400 A/m² (off-set) DC: 240-600 A/m²(off-set) AC: 400-600 A/m² (amplitude) Frequency AC: 100 Hz-10000 Hz

EXAMPLES

Crystalline structures and grain morphologies are different in cathodicdeposits of copper when using conventional vs. proposed technology.

The electro-deposits of cathodic copper obtained with the conventionaland proposed technology were carried out in double-walled glasselectrolysis cells containing the synthetic solution Cu²⁺=30 gpl andconcentrations of sulphuric acid (180 gpl), without additives to inhibitcrystalline growth. These were maintained in a double boiler at 40° C.without agitation. A copper cathode and Pb—Ca—Sn anode were used. Thedifferences in the morphology of the copper deposits and in themetalographs are presented in FIGS. 4, 5, and 6.

Example 1

Micrographs and metalographs of the cathodic deposits obtained in thecathodic reduction of copper with an imposed DC current level of 400A/m² and a sinusoidal signal of 600 A/m² (amplitude) and 1,000 Hz(frequency).

Comparison of Conventional and Proposed Technology

The conventional and proposed technologies were analyzed using themicrographs and metalographs of the cathodic deposits obtained under theoperating conditions indicated in Table 5, maintaining thephysical-chemical conditions and the instrumentation described in theexperimental test procedure.

TABLE 5 Experimental conditions Conventional Proposed technologytechnology Current DC signal DC + AC signal DC level 400 A/m² 400 A/m²AC amplitude — 600 A/m² AC frequency — 1,000 Hz. Copper Deposition 10hours 10 hours time Micrographs  20X  20X Metalographs 200X 200X

Interpretation of the Results (Example 1)

The surface quality of the cathodic deposits obtained at 400 A/m² withthe conventional DC technology is shown in FIGS. 4 a and 4 c, and theirgrain morphology in FIG. 4 e. The surfaces of these cathodes arecharacterized by abundant spherical nodules, with preferential growthconcentrated on the borders and in the centre of the deposits (FIGS. 4a, 4 c). The metalographs indicate (FIG. 4 e) fine grains with fibrousand disorganized growth.

On the other hand, the surface of the cathodes electro-won with DC+ACsignals did not present nodules and the preferential growth disappeared,even along the cathode edges, as can be seen in FIGS. 4 b and 4 d.Moreover, the metalographs (FIG. 4 e) indicate coarser grains, with thegrowth of the crystals beginning at the base of the deposits.

Example 2

Micrographs and metalographs of the cathodic deposits obtained in thecathodic copper reduction with an imposed DC current level of 500 A/m²and a sinusoidal signal having 600 A/m² (amplitude) and 1,000 Hz(frequency).

A Comparison of the Conventional and Proposed Technologies

Again, in order to compare the two technologies (conventional vs.proposed), micrographs and metalographs were examined for cathodicdeposits obtained under the operating conditions indicated in Table 6,and retaining the physical-chemical conditions and the instrumentationdescribed in the experimental procedure of the tests.

TABLE 6 Experimental conditions Conventional Proposed technologytechnology Current DC signal DC + AC signal DC level 500 A/m² 500 A/m²AC amplitude — 600 A/m² AC frequency — 1,000 Hz. Copper Deposition 10hours 10 hours time Micrographs  20X  20X Metalographs 200X 200X

Interpretation of the Results (Example 2)

The surface quality of the cathodic deposits obtained at 500 A/m² withconventional DC technology is shown in FIGS. 5 a and 5 c. These cathodesurfaces present a greater quantity of nodules than were obtained with400 A/m² DC. The size of the nodules is also greater and these aredistributed over the entire surface of the deposits, preferentially onthe borders. The metalographs (FIG. 5 e) show very fine grains andhighly disorganized spatial orientations characterized by abundantnucleation and little crystal growth. This reveals the generation ofinter-crystalline spaces that should be occupied by the electrolyte andthat result in a loss of chemical purity.

The surface of the electro-won cathodes (500 A/m² DC+600 A/m² AC; 1,000Hz) did not present preferential growth. As described in Example 1,nodules were not formed, even when operating at greater continuouscurrent densities (FIGS. 5 b, 5 d). Furthermore, the metalographs (FIG.5 e) indicate course grains with crystal growth oriented from the baseof the deposits and an excellent inter-crystalline union that does notshow the generation of interstices for electrolyte occlusion, even whenoperating at elevated current densities. This is one of the greatestadvantages of the new technology.

The greater the crystalline disorganization, the greater the number ofinterstices or inter-crystalline spaces generated in the copperdeposits. This increases the probability of electrolyte occlusion and,therefore, contamination with impurities from the cathode body.Increased crystalline disorganization results in increased cathodeimpurities and lower chemical quality, with increased penalties for theproduct on international markets.

Example 3

The morphology of the copper cathode deposits obtained withsuper-imposed current signals.

This set of tests attempts to show the effect of the amplitude andfrequency of the AC signal super-imposed over a DC current of 500 A/m².

FIGS. 6 a, 6 b, and 6 c show the micrographs of the cathodic depositsobtained when operating with DC+AC signals. In these three cases, an ACsignal (variable amplitude between 200 and 600 A/m² and variablefrequency between 5,000 Hz and 100 Hz) is super-imposed over acontinuous current (DC; 500 A/m²).

When working with an AC signal (amplitude 600 A/m², alternating signalfrequency 100 Hz), the cathodic deposits obtained had the followingcharacteristics (micrography, FIG. 6 a):

-   -   abundant amounts of small-sized nodules, and    -   a marked copper ribbon on the cathode borders.

When working with another AC signal (amplitude 200 A/m², frequency 5,000Hz), the cathodic deposits obtained had the following characteristics(micrography, FIG. 6 b):

-   -   no observed nodulation on the cathode surfaces, and    -   little preferential growth on the borders.

Finally, the last AC signal (amplitude 600 A/m², frequency 5,000 Hz),resulted in cathodic deposits with the following characteristics(micrography, FIG. 6 c):

-   -   surface with very even copper deposits and no nodules, and    -   no preferential growth on the borders.

This shows that, in order to improve the surface quality and thecrystalline growth of the copper cathodes, it is better to usesuper-imposed current signals (new technology), in which the frequencyof the sinusoidal signal plays a fundamental role. In all the casesstudied, the need to operate with AC signals having frequencies over1,000 Hz was demonstrated. On the other hand, it is necessary tosensitize the amplitude of the AC signal in order to obtain the bestresults.

FIGURE DESCRIPTIONS

FIG. 1: Equivalent Electrical Circuit in an Electrolysis Cell

This circuit considers three elements (Rs, C_(dc), Z_(f)) from both thecathodic and anodic sides. C_(dc) corresponds to a condenser of flatplates that represents the distribution of ions in the doubleelectrochemical layer. Z_(f) corresponds to the faradic impedance andrepresents the electrochemical reaction of the electrode. Rs representsthe equivalent resistance of the inter-electrode electrolytes and Rc thecontact resistances.

(A)=Anode (C)=Cathode (E)=Electrolyte

R_((C))=Electrical resistance to electric contact, (ohm)R_((E))=Equivalent electrical resistance of the electrolyte, (ohm)I_(c)=Capacitive current, (A)I_(f)=Faradic or process current, (A)C_(dc)=Capacity of the double electrochemical layer, [uF]Z_(f(A))=Anodic faradic impedance, (Ohm)Z_(f(C))=Cathodic faradic impedance, (Ohm)

FIG. 2: Stationary Characteristic I=f(V) of the Cathodic Reduction ofCopper.

A kinetic curve characteristic of the reduction of copper over astainless steel cathode is presented. It can be observed that thekinetic is slow based on the equilibrium. In this curve, the coordinatepair (V_(o),I_(o)) stands out, equivalent to the point of stationaryoperations of an electrode from a plant operating at I_(o)=300 A/m², andan electrochemical potential of V_(o)=+50 mV/SHE.

FIGS. 3 a and 3 b: Electric Current Signals Used in the Copper EWProcess: Conventional Vs. Proposed Technology

FIG. 3 shows the differences between the electric signals applied in theconventional process (DC; FIG. 3 a) and in the technology proposed forpatenting (DC+AC; FIG. 3 b).

FIG. 4: Micrographs and metalographs of the cathodic deposits obtainedin the cathodic reduction of copper with an imposed DC current level of400 A/m² and a sinusoidal signal of 600 A/m² amplitude and 1,000 Hzfrequency.

A comparison of the conventional and proposed technology, according tothe experimental conditions given in Example 1.

FIG. 4 presents 4 micrographs and 2 metalographs of cathode samplesobtained in the experimental conditions that are explained in Example 1,with the conventional (DC current signals) and proposed technology(AC+DC current signals). The micrographs of the cathodic deposits weredone with Scanning Electronic Microscopy (SEM). The experiments wererepeated 10 times and are reproduced in order to compare the morphologyof the cathodic deposits.

FIG. 4 a: Micrography 20× Conventional technology, peripheral view ofthe electrode

FIG. 4 b: Micrography 20× Proposed technology, peripheral view of theelectrode

FIG. 4 c: Micrography 20× Conventional technology, central view of theelectrode

FIG. 4 d: Micrography 20× Proposed technology, central view of theelectrode

FIG. 4 e: Metalography 200× Conventional technology

FIG. 4 f: Metalography 200× Proposed technology

FIG. 5: Micrographs and metalographs of the cathodic deposits obtainedin the cathodic reduction of copper with an imposed DC current level of500 A/m² and a sinusoidal signal of 600 A/m² amplitude and 1,000 Hzfrequency.

A comparison of the conventional and proposed technology under theexperimental conditions given in Example 2.

FIG. 5 a: Micrography 20× Conventional technology peripheral view of theelectrode

FIG. 5 b: Micrography 20× Proposed technology central view of theelectrode

FIG. 5 c: Micrography 20× Conventional technology central view of theelectrode

FIG. 5 d: Micrography 20× Proposed technology central view of theelectrode

FIG. 5 e: Metalography 200× Conventional technology

FIG. 5 f: Metalography 200× Proposed technology

FIG. 6: Morphology of the cathodic deposits in the cathodic reduction ofcopper obtained at a DC current level of 500 A/m² and with analternative sinusoidal signal of variable frequency and amplitude,according to the experimental conditions of the general procedure, andthose in Example 3.

FIG. 6 a: Micrography 20× Frequency 100 Hz, Amplitude 600 A/m²

FIG. 6 b: Micrography 20× Frequency 5,000 Hz, Amplitude 200 A/m²

FIG. 6 c: Micrography 20× Frequency 5.000 Hz, Amplitude 600 A/m²

FIG. 7: Experimental Set-Up Used to Carry Out Tests

Example Selection of Operational Parameters Copper Concentration

In copper EW plants, the copper concentration varies between 38 gpl and45 gpl. However, the experimental tests were done with values lower than30 gpl. Operating at elevated current densities and at this copperconcentration, the effect of using DC and DC+AC signals can be comparedmore clearly.

Acid Concentration

Normally, the acid concentration varies between 160 gpl and 190 gpl. Inthis case, the typical value used in the plant (180 gpl) was applied.

Temperature

On average, a constant temperature of 40° C. was used so as not tofavour the quality of the copper deposit. It should be noted that anelectrolyte at the plant enters the electrolysis cells at a minimum of45° C., in particular for high current densities DC (j>250 A/m²).

Cathodic Cycle

The copper deposition time was initially set at 5 hours in order to endthe experiments with 10 hours of copper electro-deposition, therebyobtaining thicker deposits for the metalographic analyses.

Additives

No additives were used to control the preferential growth of the coppercrystals. This was done in order to compare the effect of using DC vs.DC+AC signals on the grain morphology.

Electrolyte Flow

The experimental tests carried out for the present invention were donewithout electrolyte agitation in order to focus on the crystallinegrowth and the electrical signals and not mass transport. Strictlyspeaking, the tests were batches without agitation.

DC and AC Current Signal Ranges

DC signals between 400 A/m² and 500 A/m² and AC signals between 25 A/m²and 600 A/m² were used.

AC Signal Frequency Range

The experiments were done with AC frequencies ranging between 100 mHzand 5,000 Hz.

Materials and Reactives Reactives

-   -   Sulphuric acid H₂SO₄    -   Pentahydrated copper sulfate CuSO₄*5H₂O    -   Distilled water

Cell and Accessory Equipment

-   -   Metrohm Cell    -   Reference electrode Ag/AgCl; Reference potential: V=+200 mV/SHE    -   Auxiliary electrode    -   Thermometer    -   Cables    -   Heater HAAKE F2 120° C.

Cu Test Tubes and Accessory Materials for Conditioning

-   -   Copper wire 2.91 mm diameter    -   Synthetic resin in cold Epofix    -   Fe-silicate sandpaper    -   Alumina    -   Polishing disk

Electro-Chemical Experiment Set-Up

Electro-chemical cell

The cell used is double-walled (Metrohm brand), which requires therecirculation of hot water to maintain the electrolyte temperature at40° C. The set-up and connexion of the equipment are shown in FIG. 7.

In FIG. 7, the numbers indicated correspond to:

-   1=Metrohm cell-   2=water heater-   3=power source 1-   4=power source 2-   5=bipotentiostat-   6=signal generator-   7=oscilloscopy-   8=adding circuit of DC+AC signals-   V=voltmeter-   A=amp meter

Electrodes Used Working Electrode

A copper disk (6.65 mm²) mounted on a synthetic resin briquette was usedas the working electrode; this was screwed into a conductive support inorder to make the electrical connexion.

Auxiliary Electrode

A Pb—Ca—Sn electrode was used as the auxiliary electrode.

Reference Electrode

In order to measure cell potential, Ag/AgCl. (Vref: 200 mV/ENH) was usedas a reference electrode.

Experimental Procedure Preparation of the Aqueous Solution

An aqueous solution was prepared with a concentration of 30 gpl of Cu²⁺.For this, 400 ml of distilled water were heated in a glass ofprecipitate and 117.7 g of CuSO₄*5H₂O were added to this. Later, 65.5 mlof sulphuric acid at 98% were added. This solution was poured into agraduated matrass and left to repose for 2 hours to obtain exactly 1 L.

Experimental Set-Up

FIG. 3 is a photograph of the actual set-up used in the laboratory tocarry out the tests for this research. The following table details theequipment and instruments used.

TABLE Description of the experimental set-up No. Equipment Description 1Electro-chemical double-walled cell, Metrohm 2 Signal generator:contribution for the alternative component, AC 3 Stabilized powersource: contribution to the continuous level, DC 4 Additive electroniccircuit box, DC + AC 5 Power source, ACM to energize operationalamplifier, AOP1, of the box described in 4 6 Power source, ACM toenergize the operational amplifier, AOP2, of the box described in 4 7Water heater-recirculator 8 Multimeter Fluke, used as voltimeter 9Multimeter Fluke, used as amperimeter

Experimental Procedure

-   1. 100 ml of electrolyte were added to the cell and heated to 40° C.-   2. The briquette was mounted with the copper disk on the conductive    support and later inserted into the cell.-   3. The auxiliary and reference electrodes were inserted into the    cell.-   4. The electrodes were connected to the circuit.-   5. The null current potential was measured.-   6. The power source and signal generator were activated for the    tests with a compound signal.-   7. The selected parameters were established.-   8. After the copper deposition time, the electrode was removed from    the solution.-   9. The electrode was washed with abundant water.-   10. The electrode was dried and placed in a desiccator with    silica-gel to avoid surface oxidation.

With the present invention, specifically for the electro-winning ofcopper, the operating variables that can be manipulated externally(e.g., concentration of cupric ions in the solution entering the cells,electrolyte temperature, electrolyte flow to the cell) are establishedin function of the super-imposed DC+AC current. The chemical productionand quality of the electro-won copper depend basically on the DC+ACcurrent level imposed on the copper EW process.

For example, in the copper EW process, it is possible to operate withthe solution entering the cell at temperatures below 40° C., having Cu²⁺concentrations lower than 30 gr/l, and with flows lower than 240l/min/cell. The production and the chemical quality of the cathodesdepend fundamentally on the amplitude and frequency of the AC currentsignal super-imposed over the continuous DC current (300 A/m²).

ADVANTAGES OF THE PRESENT INVENTION

-   1. Operation at greater continuous current levels.-   2. Increased copper cathode production in industrial plants with the    same installed capacity.-   3. Improved movement of cupric ions, protons, and other ions in the    diffusion layer, toward or from the interface of the    electrode/electrolyte reactions. This allows operations with lower    electrolyte temperatures, consuming less energy and making the    process less costly.-   4. Orderly growth of the grain structure, which assures a higher    quality chemical of the electro-won and/or electro-refined cathodes.    This avoids the consumption of additives (“guar-gum” for    electro-winning; Thiourea and glue for electro-refining), making the    process less costly.-   5. Improved surface quality of the cathodes and, consequently,    decreased risk of formation and growth of nodules and entrapment of    impurities. This avoids the consumption of additives (“guar-gum” for    electro-winning; Thiourea and glue for electro-refining), making the    process less costly.-   6. Improved transfer of hydrogen ions (H⁺, or protons) in copper    electro-winning from the anode/electrolyte reaction interface toward    the bulk of the solution, improving the behaviour of the Pb—Ca—Sn    anodes in terms of their chemical stability when faced with    corrosion and their operation in electrolysis cells. This allows an    increased useful life of the Pb—Ca—Sn anodes operating in industrial    plants. The resulting achievements avoid the consumption of cobalt    sulphate, making the process less costly.-   7. Decreased production of lead slime in the cells, generating    savings in cell maintenance, resulting in less contamination and a    less costly process.

1. A method for electro-winning of metals, the method comprising: inelectrolytic cells with permanent cathodes of stainless steel or Custarting sheets, Pb—Ca—Sn anodes, and an acidic aqueous solutioncomprising approximately 180 gpl H₂SO₄, superimposing a sinusoidalcurrent over a non-zero continuous offset current to produce high-purityelectrolytic copper.
 2. A method for electro-refining of metals, themethod comprising: in electrolytic cells with permanent cathodes ofstainless steel or Cu starting sheets, anodes of impure copper fromfounding, and an acidic aqueous solution comprising approximately 180gpl H₂SO₄ superimposing a sinusoidal current over a non-zero continuousoffset current to produce high-purity electrolytic copper.
 3. The methodaccording to claim 1, wherein the sinusoidal current applied in theelectrolytic cells has an amplitude greater than 400 A/m² and afrequency greater than 1,000 Hz.
 4. The method according to claim 2,wherein the sinusoidal current applied in the electrolytic cells has anamplitude greater than 400 A/m² and a frequency greater than 1,000 Hz.5. The method according to claim 1, wherein the solution entering thecells operates with temperatures less than 40° C., with Cu²⁺concentrations less than 30 gpl, and with flows less than 240 l/min/cellwhen an amplitude of the sinusoidal current at a continuous offsetcurrent of greater than or equal to 300 A/m².
 6. The method according toclaim 2, wherein externally manipulated operating variables areestablished in function of amplitude and frequency of the sinusoidalcurrent super-imposed over a continuous offset current greater than orequal to 300 A/m².